i know the answer is3 liters - i need ot see the work so i can learn how to do the rest of my work!
asked Jan 13, 2015 in Algebra 1 Answers by jennifer

Your answer

Your name to display (optional):
Privacy: Your email address will only be used for sending these notifications.
Anti-spam verification:

To avoid this verification in future, please log in or register.

2 Answers

Let b=the volume of "neat" boric acid in litres. We can write b/(b+3)=7/10 because we added 3 l of water to make a 70% solution (70%=7/10). To find b, cross-multiply to get rid of the fractions: 10b=7b+21, so 3b=21 and b=7 l. What we want know now is how many litres do we have to add to get a 35% solution? Let's call this volume of water v. So b/(b+v)=35/100=7/20. We know b=7, so we can write 7/(7+v)=7/20. Cross-multiply: 140=49+7v. Divide through by 7: 20=7+v, so v=13 l. It appears then we need 13 litres of water to make a 35% solution of boric acid, that's 10 more litres to add to the 3 litres already added, not the 3 l you were expecting.

The only way I can see how the answer might be 6 l (3 l added), is by using proportionality: it takes 3 l of water to make a 70% solution, so to make a 35% solution (half the 70%) we need twice as much water, 6 l, which is an added 3 l. Somehow, though, I don't feel that proportionality is the right approach. I look at this way: instead of 7 litres of pure boric acid, we have 7 red balls. We add 3 balls of water making 10 balls altogether, 7 of which are red. That's 70% red balls. We now add another 10 white balls of water making 20 balls altogether, 7 of which are red. 7 out of 20 is 35%, representing 35% boric acid solution.

Which solution do you think makes the most sense? Discuss with teacher?

answered Jan 14, 2015 by Rod Top Rated User (487,100 points)
I also know the answer is 3 but I don't know how to solve it either.
answered Feb 14 by Dino Zammit Level 3 User (2,720 points)
Welcome to MathHomeworkAnswers.org, where students, teachers and math enthusiasts can ask and answer any math question. Get help and answers to any math problem including algebra, trigonometry, geometry, calculus, trigonometry, fractions, solving expression, simplifying expressions and more. Get answers to math questions. Help is always 100% free!
79,821 questions
83,639 answers
66,561 users